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The Big Unit, an overpowering lefty who last June became the 24th pitcher to win 300 games, made the expected announcement Tuesday on a conference call.

"I really wanted to go out on my terms," Johnson said. "I just feel like there's not a lot more for me to do in this game. I just think it's a natural progression when you play this long. Eventually you have to say it's time."

Johnson said he is at peace with the decision and he is 100 percent now is the right time to bow out. The Livermore, Calif. native said he enjoyed his time with the Giants, especially working with the team's young pitchers.

A five-time Cy Young Award winner, the 46-year-old Johnson accomplished just about everything in his remarkable career that a player hopes for in baseball.

He owns a World Series ring and co-MVP honors, and was a 10-time All-Star. He threw two no-hitters, including a perfect game, and ranks second on the career strikeout list.

The 6-foot-10 Johnson finishes with a career record of 303-166 and 4,875 strikeouts in 4,135 1-3 innings for Montreal, Seattle, Houston, Arizona, the New York Yankees and San Francisco. His strikeouts are the most by a left-hander and second to Nolan Ryan's 5,714.

Johnson overcame several injuries to keep pitching at a high level into his mid-40s. He said before last season ended that he looked forward to going home to Arizona and spending time with his family before making a decision about his future.

"It's taken this long into January because I definitely wanted to just kind of relax from the season being over and make sure I had a clear head when I made this decision, and that I would be making it wholeheartedly and would be sticking to it," he said.

Johnson went 8-6 with a 4.88 ERA in 17 starts and five relief appearances for San Francisco last season despite missing more than two months with a strained left shoulder that also had a tear in the rotator cuff. He returned in late September as a reliever, a role he couldn't see himself embracing in order to keep pitching.

Johnson had said part of his decision would be based on whether he thought he could be durable enough to be a starter again. As he was working his way back last year, he acknowledged that pitching more than five innings and being effective would be tough at this stage.